The invention relates to an electromechanically controllable electric resistance element. With such an element a determined resistance, and therefore in electric circuits having a constant voltage, the current or a partial voltage can be selectively adjusted. The adjusted current and/or the partial voltage can be used as a correcting variable for an electric automatic regulation and control, respectively, e.g. of a servo drive.
With the corresponding known solutions a metal, graphite or an appropriately electric conductive composite material has been deposited upon a substrate consisting of a dielectric material, wherein for this screen printing techniques with a subsequent temperature treatment have been employed, for example. Along such an electrically conductive layer a metallic contact element is reciprocated. The contact element is pushed against the surface of this electrically conducting coating with a determined spring force. With these solutions, abrasion of the electrically conducting material due to wear also occurs which is caused by the reciprocation of such contact elements, which therefore leads to a change or the respective specific resistance during the operating period as well.
That""s why in WO 00/44032 A1 it has been proposed to substitute the electrically conductive layer for a single layer which comprises at least silicon and a metal in addition to carbon similar to diamond, such that wear of the electrically conductive layer can be appropriately reduced without any additional lubricant as well.
In accordance with the teachings described in WO 00/44032 A1 such a layer is to be formed under vacuum, e.g. with an organosiloxane added, wherein in particular the content of the metal within the layer results in a reduction of the electric resistance, and the layer solely serves as an electric conductor between the mechanically movable metallic contact element and at least an electric contacting terminal.
However, the deposition of such a layer with conventional methods is only possible in a very difficult way, if at all, when a great number of such layers are to be obtained with reproducible electric properties.
If a great spreading of the electric properties of the appropriately made layers is permitted, however, it is urgently required both to calibrate each single layer equivalently and to accomplish expensive calibrations and compensations, respectively, such as with electronic means which increases the manufacturing effort of complete units adequately and influences the cost negatively.
Therefore, it is the object of the invention to provide an electromechanically controllable electric resistance element which can be economically manufactured, in particular with reproducible electric properties, and which comprises a high wear resistance and which achieves a high life period without any lubrication as well.
According to the invention this object is solved with a resistance element having the features of claim 1.
Advantageous embodiments and improvements of the invention can be achieved with the features included in the subordinate claims.
The electromechanically controllable electric resistance element according to the invention provides an electrically conductive layer per se known which is located and formed upon a dielectric substrate, respectively. The electrically conductive layer consists of a homogeneous material and is linearly shaped in a line form or is curved following a radius.
In accordance with the desired electric properties the layer is formed each with a predetermined width and thickness over the total length of the layer in order to ensure a constant specific resistance irrespective of which location a voltage tapping takes place with a contact element which is mechanically movable along the length of the layer. The thickness of the electrically conductive layer should be constant over the total length. In the normal case, this applies to the width of the electrically conductive layer as well. However, the width can also be varied over the length of the layer continuously or in bounces.
Of course, on such an electrically conductive layer there is at least another electric contacting terminal which is preferably located on a front end of the electrically conductive layer. It is possible to provide a second contacting terminal which therefore should be formed advantageously on the opposite front end of the electrically conductive layer.
The mechanically movable electric contact element is pressed with a predetermined pressing force orthogonally upon the surface of the electrically conductive layer, and is able to be moved in a translatory motion or following a circular path, as the case may be. Such a contact element can be formed from a resilient material, for example, which is bent at right angles towards the surface of the electrically conductive layer.
According to the invention, a wear resisting layer separating this layer and the mechanically movable contact element from each other is formed upon the electrically conductive layer, which is exclusively made of carbon similar to diamond and does also not contain any additional hydrogen, in contrast to the solutions known from the prior art. This wear resisting layer is in a contiguous contact with the mechanically movable contact element, and due to its mechanical properties, in particular the frictional behaviour and the achievable hardnesses, wear of the wear resisting layer does not occur although it is allowed to abandon lubricants.
It is necessary to form the wear resisting layer with a constant thickness such that, in each position of the contact element, it has a constant electric boundary resistance between the contact element and the electrically conductive layer which will be added as a constant value to the electric resistance which is determined by the effective conductor length of the electrically conductive layer between a contacting terminal and the respective position of the mechanically movable contact element.
In connection with relevantly suitable manufacturing methods which subsequently still are to be dealt with in more detail, a resistance element according to the invention can be manufactured in a great number of pieces which electric properties thereof can be maintained within close tolerance ranges in a reproducible manner, also with great numbers of pieces.
In addition to the already mentioned dimensional parameters for the electrically conductive layer with respect to the width and thickness thereof it is also allowed to influence the controllable resistance range of the resistance element by selecting an appropriate material for these electrically conductive layers.
Thus, the most different metals or metal alloys can be selected in order to provide low-impedance through high-impedance resistance elements.
However, it is also possible to employ an electrically conductive layer made of graphite carbon.
With definite combinations of material of the substrate and electrically conductive layer it may be favourable to form between them a so-called bonding agent layer, wherein in this case as well the conductivity of such a bonding agent layer should be considerably smaller than that of the electrically conductive layer. Therefore, the bonding agent layer should have insulating properties. Examples of suitable materials of such a bonding agent layer are Al2O3 or carbon similar to diamond as well.
The less expensive polymer plastics can be employed as substrate materials. Even though it is a matter of most different plastics per se known such as for example PMMA, polycarbonate, polyimides, acrylics and others which can also contain filling materials, in particular fiber reinforcements.
Such a substrate can also be provided appropriately and employed in the form of a film with electrically conductive and wear resisting layers.
However, it is particularly advantageous to select a substrate with a surface which comprises a low surface roughness, if possible, since both the thickness of the electrically conductive layer and the thickness of the wear resisting layer can be kept very thin, and electrically as well as mechanically conditioned, uniform layer thicknesses should be maintained. Thus, of course it is required to form the surface of the wear resisting layer being in a contiguous contact with the mechanically movable contact element with a low roughness to ensure favourable friction relations.
There are various ways to form the electrically conductive layer upon a surface of a substrate and to locate and fix there such a layer, respectively.
Thus, for example a metal layer can be embedded in a flush manner into a contour formed on the substrate surface, if possible, such that at least the area of a layer which will be swept during the motion of the contact element, is exposed.
In this case, for such an electrically conductive layer an adequately formed and dimensioned film can be used.
However, it is also possible for such an electrically conductive layer to be deposited under vacuum on the surface of a substrate with per se known methods in the thin film technique, wherein the layers can be deposited in an appropriate size and shape by means of photolithography processes or the use of masks.
For the wear resisting layer which is exclusively formed from carbon similar to diamond these coating methods are worthy of consideration only.
Such layers can be particularly favourably manufactured with the method known under the designation of Laser-Arc. This method is described in DE 39 01 401 C2 and DE 198 50 218 A1, for example, wherein for this application it shall be fallen back upon the disclosure in application thereof.
However, with the laser-arc method the electrically conductive layer and the already mentioned bonding agent layer, respectively, which is necessary as the case may be can also be manufactured.
To ensure the desired mechanical properties of the resistance element according to the invention it is required at least to form the wear resisting layer upon the surface of the electrically conductive layer with such a width and length ensuring that the mechanically movable contact element is already sweeping and contacting merely areas which consist of carbon similar to diamond.
However, it is more favourably to coat the entire surface of the electrically conductive layer with the wear resisting layer, and more especially it is advantageous to achieve an overlapping coverage of the external edges of an electrically conductive layer as well such that it is also able to function as an additional protective layer, in particular against corrosion.
Considering the mechanical and electric properties it is advantageous to use a mechanically movable contact element which at least provides a portion of graphite carbon, and this portion is in a contiguous contact with the wear resisting layer such that the frictional relations are improved therewith.
As is generally known, layers of carbon similar to diamond comprise SP2 type bond fractions and SP3 type bond fractions for the graphite phase and diamond phase, respectively. With these layers similar to diamond, the electric resistance behaves as well as the hardness of such a layer, which also rises with an increasing SP3 type bond fraction. Therefore, a layer made of carbon similar to diamond with a high SP3 type bond fraction is harder, more wear resisting and has a higher electric resistance as well.
However, for certain cases of application it may be advantageous to form a wear resisting layer as a so-called gradient layer but which is exclusively formed of carbon similar to diamond as well. On that occasion, the continuous gradients, if possible, will be formed through different SP2 and SP3 type bond fractions. The structure of such a gradient layer should be, if possible, such that the SP3 type bond fraction is continuously increasing with respect to the SP2 type bond fraction starting from the side of the wear resisting layer which is in a contiguous contact with the electrically conductive layer.
The wear resisting layer to be used according to the invention should have a hardness of at least 20 GPa, and more especially advantageously a surface hardness ofxe2x89xa740 GPa, at least on its surface which is in a contiguous contact with the mechanically movable contact element. The quality of the wear resisting layer can be tested by means of a non-destructive measurement of the modulus of elasticity with laser induced acoustic surface waves using a method which is known under the designation of LAwave.
A controllable electric resistance element according to the invention can be advantageously used as a location and position sensor, respectively, wherein the mechanically movable contact element will be moved into a place or position which corresponds to a definite electric resistance, and as already mentioned at the beginning, with constant voltages, for example, in order to have a definite current flown in a circuit which in turn can be used for controlling other components.